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Commitments and contingencies - Future Minimum Payments under Non-Cancellable Leases (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Rental commitments [Member]  
Operating Leased Assets [Line Items]  
2018 $ 3,440
2019 1,784
2020 232
Future minimum payments, Total 5,456
Bandwidth Lease Commitments [Member]  
Operating Leased Assets [Line Items]  
2018 11,536
2019 418
2020 84
Future minimum payments, Total $ 12,038